The present invention generally relates to a secondary air supply system in an automobile internal combustion engine having a supercharger for controlling the supply of a secondary air required by an exhaust gas purifying device.
The use of a supercharger, or a turbo charger or turbo supercharger as it is generally called, is not a new and recent development nowaday. The supercharger is known as a means effective to boost the power of the engine with a minimized fuel consumption and generally contains a compressor and a turbine linked together by a common shaft. The supercharger is usually driven by the turbine which is in turn driven by exhaust gases emitted from the engine, although on some engines the supercharger is driven by a belt or gears from the engine instead of by an exhaust-gas turbine.
For driving the exhaust gas turbine of the supercharger, the prior art is such that exhaust gases emitted from all of the engine cylinders are supplied, before they reach an exhaust gas purifying device such as a catalytic converter or an afterburner, to a turnbine chamber in which the turbine is located.
On the other hand, it is well known to those skilled in the art that the exhaust gas purifying device, particularly the afterburner or the catalytic converter of a type wherein an oxidizing catalyst or a combination of oxidizing and reducing catalysts is utilized, requires the supply of an additional air, i.e., a secondary air, to achieve an effective and efficient purification of the exhaust gases with the quantitative requirement thereof varying with the engine operating condition. In view of this, irrespective of the employment of the supercharger, most conventional internal combustion engines employ a secondary air supply pump, driven by, for example, the engine, for supplying the secondary air to the purifying device in dependence on the engine operating condition and this is exemplified by, for example, the U.S. Pat. No. 4,139,983, patented Feb. 20, 1979.
However, the employment of the secondary air supply pump in combination with the supercharger increases the manufacturing cost of an automobile vehicle and is likely to reduce the engine power output characteristic. In order to obviate these disadvantages, an improvement has been made to utilize a portion of a supercharged air, flowing towards the engine cylinders, as a secondary air to be supplied to the exhaust gas purifying device. This is disclosed in, for example, the Japanese Laid-open Utility Model Publication No. 50-137411, laid open to public inspection in 1975.
According to the Japanese publication, there is disclosed an autimobile internal combustion engine having its fuel intake port communicated through a supercharged air supply duct to a discharge port leading from a compressor chamber of the supercharger and its exhaust port communicated through an exhaust passage to the turbine chamber of the supercharger and then to the atmosphere. This publication also discloses the alternative employment of a thermal reactor and a catalytic converter, the thermal reactor being disposed on the exhaust passage whereas the catalytic converter is to be disposed on an exhaust duct leading from the turbine chamber to the atmosphere. For supplying the supercharged air as the secondary air to the exhaust gas purifying device, a secondary air supply passage is employed having one end communicated to the supercharged air supply duct and the other end communicated to a portion of the exhaust passage upstream of the thermal reactor or the exhaust passage itself in the case of the catalytic converter employed.
Although the secondary air supply passage disclosed in the above mentioned Japanese publication incorporates therein a control valve adapted to be controlled by various parameters representative of the engine operating condition, there is the possibility that, during a certain engine operating condition, for example, a low load, low speed operating condition, in which the supercharger is substantially unable to compress the air, no sufficient amount of air will be supplied to the purifying device via the engine cylinder. Considering the fact that a relatively large amount of the secondary air is required during such particular engine operating condition, the system disclosed in the above mentioned Japanese publication appears to go beyond the requirement, although the concept is sophisticated in that no secondary air pump is required.
Apart from the combined use of the supercharger and the secondary air supply system, there has also been well known to supply the secondary air by the utilization of a pulsating flow of exhaust gases emitted from the internal combustion engine. In this system, a check valve constituted by, for example, a reed valve is used so that it can selectively be opened and closed in response to the pulsating flow of the exhaust gases emitted from one of the engine cylinders to effect the supply of the secondary air. This system is quitely simple in structure as compared with the system using the secondary air supply pump and is, therefore, largely used in most automobile vehicles.
However, when it comes to the employment of the reed valve in combination with the supercharger, it has not yet been realized because there has been encountered a difficulty in accommodating two contradictory requirements. More specifically, while a portion of the exhaust system downstream of the engine and upstream of the turbine with respect to the direction of flow of the exhaust gases towards the purifying device is dominated by a relatively high back pressure and the pulsation of the exhaust gases flowing downstream of the turbine is decelerated, the supercharger, more specifically, the exhaust gas turbine, requires a positive pressure derived from the exhaust gases with minimized pulsating flow characteristic to operate and, on the other hand, the reed valve requires the exhaust gases with increased pulsating flow characteristic to operate.